The invention relates to a method of interference-tolerant heartbeat measurement, in which method the heartbeat signal of a person is measured from a suitable part of the body and transmitted as burst signals from a transmitting means to a receiver by means of talemetric data transmission, the transmitting interval of the burst signals being proportional to the measured heartbeat rate of the person.
Known telemetric transmitters of heartbeat measuring devices typically transmit a burst of about 5 kHz each time they have detected an ECG signal. The transmitting circuit consists of a simple resonance circuit, which is activated by means of transistor control. At certain intervals, for example at every fifth oscillation, a switch (transistor) in the transmitter closes and becomes conductive, whereupon the resultant current loads new energy into the magnetic field of the coil of the oscillating circuit. After a while when the switch opens, the current is switched off and the oscillating circuit is free for resonance vibration.
In heartbeat measurement, the transmitting unit transmits a signal each time the heart beats. The receiver counts the heartbeat rate on the basis of the time difference of successive transmitted signals. The method is in principle a method of time slot coding: the data to be transmitted is included in the transmission encoded in the time between the transmissions.
The known method described above is simple and reliable in circumstances free of interference. Individual disturbances may be filtered off by comparing the resultant heartbeat value to the previous results: if the new measurement result differs too much from the previous ones, it is probably caused by some external disturbance connected with the transmission/reception channel and may thus be eliminated from the measurement results.
However, if a wireless transmission of heartbeat data is performed in an environment with interference, the situation changes substantially. Thus a continuous irregular pulse sequence may arrive at the receiver, and it is difficult, often even impossible, to select the correct heartbeat signal from this sequence. Such a situation occurs easily when two or more users of heartbeat rate measurement devices are close to each other.